


Act of Nature

by SovaySovay



Category: Supernatural
Genre: Cas possessing Dean, M/M, Poor Sam, Season 11, but I think there'll be nine-ish chapters, not sure how I'm going to end this quite yet, party in Dean's brain, so things may change
Language: English
Status: In-Progress
Published: 2016-02-16
Updated: 2016-04-21
Packaged: 2018-05-21 00:34:44
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 3
Words: 4,451
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6031657
Author URL: https://archiveofourown.org/users/SovaySovay/pseuds/SovaySovay
Summary: <blockquote class="userstuff">
              <p>If the only way to save your best friend from Lucifer is to let him possess you, you do it. And if he has to stick around for a while, well, that's just the way it goes.</p>
            </blockquote>





	1. Act of Nature

**Author's Note:**

  * For [museaway](https://archiveofourown.org/users/museaway/gifts).



> Happy birthday, muse! Hope this adds to an already-good day.

_the wind came 'round and blew this place apart_  
_it's you and me now sitting in the dark_  
_the lights are out and everybody's home_  
_it's you and me and we are both alone_

_act of nature, act of God_  
_raging through our sedentary lives_  
_we are on the brink, we are foundering_  
_spinning in this dark and rising tide_

Lucifer laughed. Seeing Cas’ face broken open into that wide smile was entirely too reminiscent of Leviathans for comfort. When was the last time Dean had seen Cas, real Cas, smile properly? Never? He couldn’t remember. The thing that wasn’t Cas stared out through Cas’ eyes and took a step towards him with Cas’ legs. How could he ever not have noticed how wrong he was? There was no sincerity in his voice, no purpose in his stride, and most distinctly, there was none of the glow in his eyes that Dean had grown so accustomed to seeing.

 _Are you sure?_ said the deeper, gentler voice in Dean’s head.

 _How many times do I have to say it?_ he thought back, trying and failing to stay calm. _Yes!_

Lucifer’s eyes widened in surprise as a thin reed of blue-white grace snaked out of his mouth, curling through the air towards Dean. There was a flash of light that blinded him and the world turned icy.

* * *

By the time Sam finally shoved open the door of the closed-up room, Lucifer was gone and Dean was lying on the floor, out cold.

“Dean?” He rushed towards his brother, wondering for the millionth time in his life if he was dead.

Dean opened his eyes.

“Dean?” Sam repeated, even more worried now. Dean sat up with an unusual amount of abruptness, accepting Sam’s help in getting to his feet. He lifted his hands to his eyes, examining them carefully. After a minute’s study of his fingers and palms, he finally looked away and up at Sam.

“I’m sorry, Sam,” he said, and his voice was clearer and softer than it had been before. “This was the only way to save me, and Dean insisted.”

Sam frowned and blinked a couple of times.

“Cas?”

Dean-- Cas nodded.

“We should, uh, get you back to the bunker.”

“Yes, that would probably be wise.”

* * *

The drive back was quiet and vaguely disagreeable. Sam drove, his eyes flicking occasionally to his right, unnerved each time by the calm apprehension in his brother’s countenance. No, not his brother. _Cas._ He hoped they’d figure this out fast, at least so he wouldn’t have to remind himself who was sitting next to him every time he looked to the side. He cleared his throat and Cas looked at him.

“You’re uncomfortable,” Cas said matter-of-factly.

Sam grimaced slightly. “Yeah. I know I probably shouldn’t be. Of any angel to be piloting one of us, you’re the only one I’d trust to do it. It’s just--”

“Dean not being Dean is wrong.”

Sam didn’t answer, possibly out of a reluctance to offend Cas; after all, _wrong_ was a pretty heavy concept.

“You know,” Cas said after a few minutes, “Gadreel was able to be dormant most of the time, when you were...”

Sam glanced at him, raising his eyebrows. “Do you think you’d be able to--?”

“I don’t see why not,” Cas replied. “I can certainly try, or at least try to speak to Dean.” Sam shook his head and smiled.

“What?” Cas frowned.

“Nothing,” Sam said. “Just... both of you awake in there? That’s gotta be a mess.” Cas nodded in understanding, the corners of his mouth turning up slightly.

“Yes, it’s--” Cas went silent, head turned out the window.

“Cas?” There was no reply. Had he traded consciousness with Dean? Doubtful; Dean wouldn’t shut up like that. “Dean?” Nothing.

The rest of the drive passed in silence.

* * *

“Just... both of you awake in there? That’s gotta be a mess.” Dean heard Sam’s voice, but it was slightly muffled. That probably made sense, actually-- hearing through someone else’s mind? Weird. That someone else being Cas? Weirder. He could tell that he wasn’t the one in control of his own limbs or his speech, but he still felt awake. Like maybe, if he tried, he could talk.

“Yes,” he heard himself say outside his own head, “it’s--”

 _Cas!_ he tried saying. His outside voice stopped abruptly.

 _Dean?_ Well, that worked. The other voice in his head was definitely Cas’, and it was definitely talking back.

_Were you telling the truth, about being able to let me take the wheel?_

_I believe so. You should have the ability to control your own body. I just have to let you._

_Awesome. Sammy probably thinks something’s wrong; you just stopped talking._

_Oh. That wasn’t intentional._

_He won’t mind._

_Do you want to talk to him?_

_When we get home. I’m exhausted._

_That’s normal. Being a vessel isn’t easy. Dean, if you want me to go--_

_Cas, shut up._

* * *

When Sam looked back up the bunker’s steps where Dean was walking in from the car, he could tell immediately that it was, in fact, his brother, and that Cas was either politely allowing them to talk or else resting.

“Hey, are you doing alright?” Sam asked carefully, trying to assess Dean’s mental state without staring at him. Dean descended the steps slowly and ran a hand over his face, then rubbed at his eyes.

“Yeah, I’m just tired.”

“Really? Because you’re splitting your brain and your body with Cas.” Dean forced a laugh.

“I’m not splitting my brain with him, Sam. He’s not picking through my head.”

“I’m sure he’s not, but it’s not like you’re used to sharing all _that_ with anybody else.”

Dean opened his mouth as if to make a scathing remark, then lost the trail of his thoughts and retreated to the kitchen.

 _Cas,_ he thought.

 _Yes?_ The reply was instantaneous.

_You’re not, uh, digging through my thoughts, right?_

_Of course not, Dean._

Dean sighed in relief, then immediately felt slightly guilty.

_Cas, not that I don’t trust you, but is there a way for me to keep some of my thoughts private?_

Cas said nothing, and Dean could practically see his head tilted quizzically, his eyes narrowing.

_It’s just... you know me, Cas, I’m not so great with the whole “emotionally open” thing._

_Of course, Dean. I suppose you could imagine shutting doors in front of things you don’t want me to see. If I encounter one, I’ll turn away and ignore it._

The dark and endless space in which Castiel stood was silent now, Dean’s thoughts gone. A door appeared, followed by another and another and another and another, until he stood in a labyrinth of locked doors. Cas sighed, putting a hand carefully against the frame of one.

* * *

Sam sat in the library, books lying on the tables, their cracked spines holding together for one more read-through. The paper crumbled under his fingers occasionally, but most of the Men of Letters’ library was in good condition. Dean walked out of the kitchen and dropped a beer on the table in front of Sam.

“You seem like you’re in a weirdly good mood.”

“Yeah, well,” Dean shrugged, settling into a chair. “Why shouldn’t I be? We saved Cas from Lucifer, so we’re done. Just let him charge off and fight Amara-- not our problem right now.”

Sam scoffed lightly and went back to his book. Dean hummed to himself, drummed his fingers on the table, and stood up. Sam looked up at him, confused.

“Forgot something in the car,” Dean muttered, then walked out of the library. Sam stared after him, then shook his head and found his place on the page.

Dean opened the Impala’s door and peered inside until he spotted his jacket lying on the floor. He picked it up and folded it over itself, then shut the door, catching a glimpse of his own reflection in the dark window. His eyes flashed blue and he stood taller, but his gaze remained trained on his own reflection.

 _You okay, Cas?_ Dean whispered into the dark labyrinth of his own locked thoughts.

Silence.

A flicker of a heartbeat.

A new door appeared near Dean, one that he had not created.

Cas turned on Dean’s legs and walked them away from the car, the window, and the reflection.


	2. Of Angels And Angles

_there are angels in your angles_  
_there's a low moon caught in your tangles_  
 _there's a ticking at the sill  
_ _there's a purr of a pigeon to break the still of day_

 _as on we go drowning, down we go away_  
_and darling, we go a-drowning_  
_down we go away_  
_away_

It was cold in the mornings, especially in the bunker, where there was no heat or comfortable surface (besides the beds) to speak of. Dean’s room was icy, and that bed was empty, no left-over warmth from someone having slept there the night before. No angel needs to sleep, after all, and throughout the long hours of the night the two souls had circled each other in the darkness, winding around Dean’s bolted thoughts and feelings. A day sharing his mind, eight years knowing him, forty years to find him in hell, thirty years watching over him, and millennia anticipating him, but there still wasn’t a single one of those doors that Castiel could open and feel confident he knew what was behind it. Dean, for his part, wandered the labyrinth of his own head with more apprehension than usual, if that were possible. He knew Cas wouldn’t intrude on his thoughts, but it was instinct more than anything else that made him slam the doors and lock them. So there in a chair by the desk sat the body, perfectly still while the mind churned and vied for control, until the early morning came and the room turned cold.

It was there, of course, that Sam found them in the morning. For a moment, Dean looked like Dean again, turning his head quickly as the door opened to glance at Sam. Then he stood and stared into Sam’s eyes unwaveringly, seeming to regain the four inches of height to tower over him, and Sam blinked, remembering who he was looking at.

"Hey, Cas," he said carefully.

"Good morning, Sam."

"Would you mind letting Dean have the wheel for a while? There’s something I wanted to talk to him about."

"Of course." Cas’ eyes gleamed blue, and Dean was Dean again, slightly slumped and animatedly frustrated.

"You doing okay in there?" Sam eyed him warily.

"I will be once you stop asking me that." Dean avoided eye contact and pushed past his brother, stalking out to the library in long strides. Sam tried to keep up, feeling like a kid.

"Seriously, Dean, you need to tell me if you’re not okay. What if we need to find another vessel to hold Cas?"

Dean stopped short, turning around with an exasperated look on his face, his arms flying up in an exaggerated gesture of _no shit, Sam._

"Of course we need to find another vessel for him! This isn’t permanent--" he frowned and paused momentarily, as if he was sifting and organizing his thoughts. "It’s not," Dean said, glaring back up at Sam before dropping into a chair at the table. Sam sat across from him, trying to assess what was happening in his brother’s head.

"Anyway," Dean snapped, "I want my own damn body back. This is way too weird, even for us. I didn’t actually sleep last night, I’m pretty sure. I-- or Cas, I guess, he just sort of sat there in the dark for hours." He paused again. Is that what it’s always been like for Cas? When he first met the angel, he was always showing up in the middle of the night or watching Dean sleep, weird things like that… had he just been bored? No, impossible. When you’re a bored millennia-old angel, there must be an endless list of things better to do than visit Dean Winchester.

"Dean, you need to consider the options in case we can’t find a new vessel for him right away." Sam’s voice broke in on Dean’s train of thought. "I mean, this could last for a long time, even if it isn’t permanent. Have you talked to Cas about it?"

"What? No! I mean, this is a bad situation for everybody involved. I haven’t _discussed_ it with him or anything, but there’s no way he wants to stay in here. He probably wants to get out as bad as I want him gone."

"You really want him out?"

Dean stared, nonplussed, at his brother’s sincere face.

"Of course I don’t want to be splitting my body with Cas! Come on!" Sam raised his hands in a gesture of defeat.

"I just thought this might not be such a bad thing-- you guys have been through a lot in the past, what, eight years? You don’t talk about anything with him."

"And? We’ve been through a whole lot more in the past thirty-some years. You don’t hear me complaining."

A smile twitched at the corners of Sam’s mouth and he drummed his fingers on the table. "You should ask him what he thinks, Dean. We can’t just assume we know."

"I’m pretty damn sure he’d like his own body back."

"Really?"

"Yes! What’s with the third degree?"

"Did you not notice you’ve basically been holding hands with yourself for the past three minutes?" Dean looked at his hands. Sam was right. His fingers were interlaced, but when he tried he could only shift his right hand; the left’s fingertips pressed into the back of his right hand.

“Weird,” Dean muttered. Reassuring, but _weirdly_ so. Sam watched him as his chin sank against his chest and he stared at his locked hands. After a minute, he realized Cas must’ve piped up, and neither of them would be surfacing for some time. He stood and went to the kitchen with the intention of making something for breakfast, but quickly remembered that he could go back to sleep, and so he did.

* * *

 _Dean?_ Cas said into the darkness.

 _Yeah, Cas?_ Dean replied, still uncomfortable with the way Cas’ voice came from all around him, both outside and within his brain. _Hey, I’ve got to ask you a question._

Cas did not answer, so Dean continued. _The way when one of us talks in here, the other can sort of hear it everywhere. Is that what it’s like when someone prays to you?_

_Well, you’re technically an angel now. If someone were to pray to me, you would hear it. Of course, there probably isn’t a soul alive who would pray to me, except for you._

_Yeah, but… Is this what it sounded like, when we were separated and I prayed?_

_Not quite. I suppose I’d liken it more to a skein of thread. That’s how I could find you-- I would follow the thread to the source. Sometimes, before the fall, being an angel was like living in a spiderweb. Humans don’t seem to pray to a particular angel very often, so there were thousands, even millions, of thin strands of prayer stretched across the universe. I always tried to pick out the ones that seemed warmer and more genuine. They led to good people, people I could help._

Dean scoffed. _I don’t know why you kept showing up here, then. That doesn’t sound a lot like me._

_On the contrary, Dean. You are certainly a good man._

Even inside the dark hallways of his mind, Dean felt his face reddening slightly.

_Look, Cas, did you hear what Sam and I were talking about?_

_Yes. You were wondering how to find me a separate vessel so that you could have your body back to yourself._

_Yeah. Sammy said I should, uh, talk to you about it. Ask you what you wanted._

_Oh. I suppose whatever’s best for you and Sam is best for everyone concerned; if you think we should find me another vessel, then we will._

_Well, yeah. We need to find Lucifer and get him the hell out of your body--_

_It’s not really my body, Dean. It’s a reconstructed copy of Jimmy Novak’s body._

_Whatever, Cas, it’s been yours since the apocalypse. But we need to get it back. Lucifer wandering around in your vessel, killing people, doing God knows what else… it’s just plain wrong._

_Well, I will grant that I miss that vessel. I have been in it longer than any other, as far as I can remember. And I know you must hate this, being so locked up inside your own head._

_Haven’t been much elsewhere in the last thirty-seven years-- kind of nice to share this mess with somebody, actually._

From the darkened hallway in which Cas stood, he heard a sharp click. He turned to look at where the sound had come from; one of the doors hiding Dean’s thoughts had opened the slightest bit, and a sliver of buttery gold light fell at his feet. He walked closer and pushed the door gently. After a moment’s resistance, it yielded to his hand.

**Notes for the Chapter:**

> Chapter 3 coming soon! Plot developments next time! Advancing the more relationship-esque storyline! This will not be an entirely happy fic!


	3. Lost My Driving Wheel

_my car broke down in Texas_  
_she stopped dead in her tracks_  
 _just called to tell you that I need you_  
_just called to tell you how I feel_   
_I feel like some old engine that's lost my driving wheel  
_ _I feel like some old engine that's lost my driving wheel_

For the next week, Sam trod carefully around his brother and the angel. Unless one of them (whichever happened to be in control at the time-- it was maddening) sought him out directly, he tried not to get in their way, preferring to bury his head in books or netflix marathons. It wasn’t precisely that he didn’t want to be around them. Quite the opposite, really; he was fascinated by the way Dean (or what used to be Dean) would freeze in the middle of talking, reading, or even sometimes walking to have a silent conversation with the other person in his head. Besides, it was impossible not to take notice of how careful Cas was when he was in charge. Sam was used to seeing Dean have little to no regard for himself, whacking his head if he stood up too fast, brushing off the occasional stubbed toe or jammed finger. But Cas was inordinately gentle when he moved; he gave a wide berth to anything that he could bump into, walked slowly through the bunker, avoided low doorways-- like anything that could possibly damage Dean’s body in the slightest way would kill him. Cas seemed to almost have wings again, so gingerly and softly would he move around the bunker.

It was easy to tell when Cas had the wheel. He still looked like Dean, but he stood straighter and walked with more purpose, his actions more deliberate and precise than the real Dean. Sam almost wanted to take a video of the transition between the two so that Dean could see it if--when--when they were separate again. Watching Cas/Dean (Dean/Cas?) stride through the bunker and then stop abruptly to slouch again and assume an expression on his face. There was another thing Sam had noticed: Cas’ face looked neutral on the surface, but there was enough emotion in his eyes to tell the difference between him and his siblings-- most other angels were pretty vacant below the surface. Maybe it was the time he’d spent as a human.

Whatever it was, it took no effort at all to notice on a rainy Saturday afternoon when Cas turned into Dean as he walked through the war room. Dean blinked a couple times, stretched his arms, and pulled a book off the shelf.

* * *

Cas stood in front of the door he had pushed open. The same golden light he had seen before filled the room, and he could just barely discern a voice whispering from inside. It sounded vaguely like Dean’s voice, but that could also be coming from just outside his head, surrounding the hallways as the brothers spoke outside. Cas tried not to eavesdrop, but it was difficult to completely avoid listening in occasionally, especially if it was Dean speaking. After all, Castiel had been listening to and answering Dean Winchester’s prayers for nearly eight years (a blip in his lifespan, but somehow also an eon), and it was practically instinct to listen attentively to what he had to say. As far as he could tell, the brothers were trying their best to anticipate Lucifer’s next move, to figure out where they could intercept him and get Cas’ old vessel back. Cas brushed it off and stepped into the room.

Music.

Ten thousand frequencies rose up in a chorus around him, the different chords and keys creating a dissonant ringing in his ears, but each one still maintaining its own melody and tempo. If he concentrated, he could just barely single them out, finally catching the thread of a song and following it like a prayer. Cas started to recognize songs he’d heard before, songs Dean had played in the car on long drives, but they were different here in some way. At certain notes or lyrics, it almost seemed like wind blew through the room, but the wind was emotion made tangible. He felt waves of joy during particular guitar riffs, a peculiar sadness in strains of Hey Jude, and countless tiny flickers of emotion that he suddenly recognized as Dean’s own feelings when he listened to his music. Cas smiled softly and shut his eyes, letting the melodies swirl around him.

* * *

Castiel took control again after Sam and Dean had had dinner, waking up and going directly to the library, where he spent the following three hours (much to Dean’s discomfort when he tried to stand and stretch later). Inside his own head, Dean stared frustratedly at the Door That Wasn’t His. It was a plain door; he’d probably kicked down thirty or more just like it. A light blue door with white trim and a silvery handle. The four small square frosted windows afforded no real insight as to what lay beyond the threshold.

Dean gave it an experimental push. Besides a slight glimmer of heat (was it really heat? He was inside a dreamscape in his own head. Did it really matter?), nothing happened. He frowned, considering whether or not he should be trying to break into Cas’ _only_ barricaded thoughts, especially since, as far as he knew, Cas had done no such thing to him.

 _Fuck it._ Dean tried the door’s handle. For a moment it didn’t budge, but then, almost like it recognized him, it relented and--

He opened his eyes, his real eyes, back at the wheel.

_Jeez, Cas, warn me next time you flip the switch, okay?_

_Sorry,_ Cas’ voice whispered around the interior of his skull like a gentle flame. _I had a question for you._

_Yeah?_

_One of the doors you created opened yesterday. I may have gone inside._

_Oh._

_Are you angry?_

Dean imagined he might have been, if he hadn’t just been about to walk through Cas’ own door.

_Nah, Cas._

_All right. Do you mind if I go back again?_

_Well, sure, if it’s that interesting. Don’t know why it would be, but sure._

_Thank you._

Dean stood and stretched his arms (an ordeal, truly), and went to find Sam.

* * *

Cas picked through the songs in Dean’s head carefully, trying to find something he hadn’t heard before. A voice caught his attention. It was low and rolling, reminiscent of mountains and rivers. Cas could feel Dean’s affection for the tune-- an almost guilty enjoyment, for some Reason unknown to Castiel. It sounded similar enough to other songs in Dean’s catalogue, but the feeling attached to it was clearly unique. When he really focused on it, he almost saw images playing alongside the song: long stretches of an empty highway at night, eyes that seemed familiar but that he couldn’t identify, and a silhouette that he just barely glanced. If what he was seeing was what Dean thought of when he heard this song, it could have been said that Dean was trying to hide his own thoughts from himself. Which, Cas thought, was entirely possible. “Heart choice” or no, his friend had never been particularly honest with himself.

Cas closed his eyes and pulled the song closer, like a coat. It was, impossibly, a song that felt like night driving, and that felt like Dean.

After that, every time Cas found himself stuck inside the headspace he shared with Dean, he lifted the same song from the recesses where it was hidden, like he was dusting off the same record over and over again to listen to it. Each time, he tried to focus on Dean’s feelings about the song, to see if he could pick out whose eyes, whose silhouette, who the other person in the conjured-up car was. He couldn’t, however, so after a point he gave up and enjoyed the tune for what it was.

In his eight years on earth, Cas had never properly understood the emotional connection Dean had to certain songs, but he thought this was about as close as he was going to get. The satisfaction when the chorus changed at the end to a truer version of itself; the building momentum from guitar to a full band; how it slowly advanced from a gentle phone call to a nearly desperate declaration of love. It lifted and simultaneously broke Castiel’s heart, which was (he assumed) precisely what it was meant to do.

In short, he loved it.

* * *

It was alright, wasn’t it? Cas had gone into one of Dean’s doors, so…

Yes. Absolutely, it was perfectly okay for Dean to open that door and saunter right in.

So he did.

And he was utterly unprepared for the explosion of color that burned behind the plain blue door. He couldn’t comprehend all of it, wouldn’t have even been able to if every color had been in a spectrum visible to human eyes. Bright, dark, warm, cool, muted tones and electric, he couldn’t say. They moved too quickly and in greater numbers than he could process or count. After two or three seconds, his eyes felt like they were on fire and he ran out, slamming the door behind him and leaning against it, beathing heavily.

Dean shut his eyes tightly and pressed the heels of his hands into them until the remaining lights popping behind his eyelids were gone. He took a deep breath, walked away from the door, and tried to forget about it.

**Notes for the Chapter:**

> I highly recommend checking out the song this chapter shares a title with-- I picked it for a reason (besides, it's the song Cas is listening to).  
> Speaking of music-- thank you SO much to Julia for her gorgeous Destiel/SPN Spotify playlist, which I listened to nonstop while writing this chapter. Her music is basically the only thing keeping me writing right now.


End file.
